This invention relates to television signal generating apparatus, and especially to apparatus for generating the subcarrier frequencies for the SECAM television system.
In the SECAM television system, the color difference signals R-Y and B-Y are frequency modulated on two different subcarrier frequencies and transmitted separately on alternate horizontal lines of the video picture. The two subcarrier frequencies are specified in the SECAM television standards to be even multiples of the horizontal line rate of 15625 Hz, thus D.sub.R (R - Y) is established at a frequency of 282 .times. 15625 Hz or 4,406,250 Hz and D.sub.B (B - Y) at a frequency of 272 .times. 15625 Hz or 4,250,000 Hz. These two subcarrier reference frequencies D.sub.R, D.sub.B are used to set the center frequency of the FM oscillator for alternate television lines during the horizontal blanking interval.
The most common prior art methods of generating the two subcarrier frequencies generally utilize a sharp, narrow pulse of approximately 0.1.mu.sec duration occurring at the horizontal line rate to drive a pulse transformer for generating a wide spectrum of harmonics of the horizontal line frequency. Two crystals, one resonating at 4,406,250 Hz and the other at 4,250,000 Hz, arranged in a bridge configuration, filter the desired frequencies from the spectrum of harmonics of the horizontal line frequencies. Although of apparently simple design, this prior art method of subcarrier generation has several disadvantages in operation and signal quality; i.e., the desired frequencies at the output of the crystal filter are only approximately 12 to 15db higher than the unwanted harmonics of the spectrum and may vary in amplitude during the line duration due to the difficulty of harmonic generation based on the relatively low horizontal frequency rate and the critical width and shape of the driving pulse waveform. The addition of an amplifier/limiter combination to improve the quality of the derived subcarriers frequently introduces additional noise and distortion to the subcarrier signals generated in this manner.